In general, in the descriptions that follow, we will italicize the first occurrence of each special term of art that should be familiar to those of ordinary skill in the art of low power current reference design. In addition, when we first introduce a term that we believe to be new or that we will use in a context that we believe to be new, we will bold the term and provide the definition that we intend to apply to that term. In addition, throughout this description, we will sometimes use the terms assert and negate when referring to the rendering of a signal, signal flag, status bit, or similar apparatus into its logically true or logically false state, respectively, and the term toggle to indicate the logical inversion of a signal from one logical state to the other. Alternatively, we may refer to the mutually exclusive boolean states as logic—0 and logic—1. Of course, as is well known, consistent system operation can be obtained by reversing the logic sense of all such signals, such that signals described herein as logically true become logically false and vice versa. Furthermore, it is of no relevance in such systems which specific voltage levels are selected to represent each of the logic states.
Power consumption has become a key problem for circuit designers with the proliferation of battery-powered devices. Circuit topologies that support power reduction are extremely valuable in extending battery life. Reference current generators are present in virtually any integrated circuit since all analog electronics require a bias current for proper operation. This reference current is also generally temperature-compensated such that the current is substantially insensitive to temperature or proportional to absolute temperature (“PTAT”) or complementary to absolute temperature (“CTAT”). Most reference current generators draw significant power due to the heavy use of saturated transistors and relatively small resistors.
Reference currents can be generated in a wide variety of ways. Two prior art examples are shown in FIG. 1 and FIG. 2. In one such prior art example of a reference current generator circuit 10, shown in FIG. 1, a voltage from a reference voltage generator (e.g., a bandgap reference voltage generator) 12 can be amplified using buffer 14 and applied across resistor 16. Bandgap reference 12 and resistor 16 are both reasonably temperature insensitive and can be tuned to achieve a desired temperature sensitivity (e.g., zero temperature sensitivity, PTAT, CTAT). However, reference current generator circuit 10 consumes considerable power. Voltage generator 12 draws significant power; nominally on the order of one microamp (1 μA). The combination of a large voltage combined with a relatively small resistor results in excessive current draw. Assuming a typical bandgap reference voltage of 1.25V and a typical on-chip resistor of 100 kΩ, reference current generator circuit 10 consumes a reference current of 1.25/100e3=12.5 μA. This current is well in excess of limits imposed by many modern battery-powered devices.
Similarly, the structure shown in FIG. 2 is also typical in that good temperature sensitivity can be achieved. However, the active devices in reference current generator circuit 18 are operated in the saturation region and will typically draw much more than 1 μA of current.
Given the wide use of current reference generators and the significant power demands of these circuits, we submit that what is needed is an improved method and apparatus for an ultra-low power temperature compensated reference current generator. Such a method and apparatus is important for use in power sensitive systems such as battery-powered electronics.